This invention relates to antennas for direction finding radar receivers and particularly to improvements for such antennas to correct for pointing inaccuracies caused by cross polarization between the receiver antenna and a transmitting antenna.
An antenna for a direction finding radar receiver usually comprises a parabolic dish pointed in the direction of a radar signal source which can be a radar transmitting element. Known receiving antennas include an array of receiving elements, usually four dipole elements, about the parabola focal point. The signals intercepted by the dipoles are combined to generate a sum signal, a delta elevation signal and a delta azimuth signal. A fifth dipole element can optionally be mounted at the boresight axis. The sum signal can be obtained from this element and the delta elevation and delta azimuth signals derived from the other dipole element are used in a known manner to maintain the boresight axis of the antenna through the transmitting element or target so long as polarization of the transmitted signal corresponds to the receiving antenna polarization. Specifically, the delta elevation and delta azimuth signals are switched and phase shifted and further combined with the sum signal to simulate a single signal being sequentially lobbed about the true array boresight axis. The amplitude modulation of this combined signal is used to provide tracking error information to the tracking servo elements of a tracking radar. So long as the target being tracked produces radar signals which are properly polarized with respect to the receiving antenna elements, the tracking radar will align itself to hold the target on the boresight axis. If, however, the target signal becomes cross polarized with respect to the receiving antenna, the apparent location of the transmitting element in space will be found to have shifted causing the radar antenna to be driven so that its boresight axis no longer intercepts the transmitting element. The result of the apparent shift of the transmitting element in space is termed pointing or positional inaccuracy caused by cross polarization products. It is the major object of this invention to provide means for compensating for this inaccuracy.
The means used to compensate for positional inaccuracy comprise sets of metalic rods spaced about the receiving dipole array with one end of each rod butt fastened to the dipole array ground plane, the rods extending generally parallel to the antenna boresight axis between a dipole array ground plane and the parabolic dish, with the other end of the rods butt fastened to the parabolic dish and evenly spaced about the boresight axis. It has been found, however, that if the rod ends at the dipole array ground plane, which is fixed with respect to the parabolic dish, are not evenly spaced about the boresight axis but are instead circumferentially adjusted, the positional inaccuracy caused by cross polarization products can be eliminated or its effect greatly attenuated.
The invention has particular utility for equipment used in meterological studies where an airborne balloon has suspended therefrom a radar transmitter including a transmitting antenna which is generally polarized with a ground located direction finding radar which tracks the balloon as it is borne on air currents to thus provide certain meterological data. If the pendulous balloon-borne transmitter should swing with respect to the balloon, as is normally the case, the transmitting antenna element, which is not normally stabilized, will become misaligned with respect to the receiving antenna producing the aforementioned positional inaccuracy caused by cross polarization products. If no means are provided for compensating or eliminating these inaccuracies it is possible under certain circumstances for the resultant errors in the derived meterological data to exceed accepted norms.